1. Field of the Invention
This invention relates to a novel and improved method for measuring a physical parameter at a first location which is remote from a second location. This invention also relates to a novel and improved sensor device for measuring such a physical parameter. In particular, this invention relates to a measuring method and to a sensor device which incorporates fiber optical signal transmission concepts.
2. Description of the Prior Art
Fiber optical signal transmission has become a significant technological innovation in electronic systems during the last years. Among the reasons for preference of fiber optical signal transmission to the former electric signal transmission is the availability of additional band width combined with the elimination of electromagnetic interferences. Other reasons include the convenient separation of the electric potentials of various system components.
Fiber optical transmission has been used for medium and long distance communication and for digital data transmission in certain industry applications where electromagnetic interference or electric separation is of special significance. Fiber optical transmission lines have also been used in connection with high voltage switching, power plant control and process control of electric furnaces. The application in aircraft and in automobiles is discussed.
In control systems signal communication in digital form is performed not only between data processing units but also in communication with input and output devices. Special signal sensitivity exists between sensors and data processing devices. Therefor, fiber optical communication on digital basis has been used for signal transmission between sensors and the control system (Control Engineering, February 1979, pages 30-33). The sensor devices serve to acquire input information relative to physical parameters which are of significance for the control system, such as temperature, pressure, position, flow, speed, etc.
In order to obtain a sensor device of high accuracy, it is advisable to convert the physical parameter to be measured directly into an optical signal and to transmit this signal through an optical fiber transmission line to the evaluation side of the electronic system. For this purpose sensor devices may be designed so that they do not require an energy source on the measurement side. Such sensor devices work entirely on optical principles and operate strictly with fiber optical signal transmission. These sensor devices may be termed passive sensors. They have the inherent benefit of high protection against electromagnetic interference and system simplicity.
Sensor devices may also be designed so that they utilize an auxiliary energy source for the sensing function in order to arrive finally at optical signals which may be transmitted through the optical fiber transmission line. The energy source may be an optical, an electrical or any other source, while the sensing principle and the signal transmission are still of optical nature. Such devices may be termed active devices.
In the European Patent Application Publication No. 10 221 is disclosed a fiber optical measuring apparatus which is used for the measurement of current or voltage. An electrical measurement signal is converted into mechanical motion which modulates a light beam. This measurement apparatus strictly makes use of amplitude measurement and is therefore subject to environmental disturbances.